Problem: 5 pencils cost $9.50. Which equation would help determine the cost of 10 pencils?
There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 5 pencils. We want to know the cost of 10 pencils. We can write the numbers of pencils as a proportion: $\dfrac{5}{10}$ We know 5 pencils costs $9.50. We can let $x$ represent the unknown cost of 10 pencils. The proportion of these costs can be expressed as: $\dfrac{\$9.50}{x}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{5}{10} = \dfrac{\$9.50}{x}$